Vehicular variable steering ratio control device and control method thereof

ABSTRACT

An operation torque estimating portion estimates an operation torque Th of a driver based on an input angle θh and an input torque T output from a torque sensor. A non-holding state detecting portion determines whether the driver is holding a steering wheel by comparing an absolute value of the operation torque Th and a predetermined value ε 1.  An equilibrium detecting portion determines whether an equilibrium point is reached by comparing the absolute value of the input torque T with a predetermined value ε 2.  When it is determined that the driver is not holding the steering wheel and the equilibrium point is reached, a switching portion switches to such a position that causes a target steering adjusting portion to multiply a target output angle θ pm by a predetermined gain G according to the input torque T so that the target steering angle is made smaller.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2005-319512 filed onNov. 2, 2005, including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a steering control apparatus of a vehicle thatincludes variable steering ratio controlling means for changing arelationship between the steering angle of steered wheels and thesteering angle of the steering by applying a driving force to thesteering input.

2. Description of the Related Art

A steering control apparatus (VGRS: Variable Gear Ratio Steering) thatincludes variable steering ratio controlling means that can change theratio between the steering angle of a steering wheel and the steeringangle of steered wheels is known (for example, refer to Japanese PatentApplication Publication No. JP-A-H11-321684). This technology aims atrealizing desired steering response under various operational conditionsof the vehicle by, for example, changing the steering ratio according tothe vehicle speed. To realize such variable steering ratio controllingmeans, for example, a transmission portion such as a gear is provided ata connection point between the input shaft in the steering wheel sideand the output shaft in the tie rods side, and the relative rotationangles of the input shaft and the output shaft are changed by applyingdriving force to the transmission portion by an actuator. It should benoted that the actuator that applies the driving force to thetransmission portion may constitute the transmission portion. Note thatfor such steering ratio control there is a method in which feedbackcontrol is performed so as to achieve a target steering angle.

In feedback control for variable steering ratio control as describedabove, the accuracy of position control is improved by setting thecontrol gain high. However, when the driver is not holding the steeringwheel (will be referred to as a “non-holding state” where necessary),the reaction force from the actuator is not received by the steeringwheel side, and therefore the control system becomes unstable. As aresult, the steering wheel may vibrate. Such a non-holding state mayoccur while the steering wheel is being returned to the neutralposition.

In the technology disclosed in Japanese Patent Publication ApplicationNo. JP-A-H11-321684, for the purpose of reducing vibration when thedriver is not holding the steering wheel, such a non-holding state isdetected based on the torque that the driver applies to the steeringwheel, and when it is detected, the control gain is reduced.

However, when the control gain is reduced in this manner, the accuracyof position control and the steering response are deteriorated, and thusthere is the problem that the positional deviation due to externaldisturbances becomes large.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a vehicular steering controlapparatus and a control method thereof that can suppress vibration whichoccurs when the driver is not holding the steering wheel, while reducingthe positional deviation due to external disturbances.

A first aspect of the invention relates to a vehicular steering controlapparatus including: (1) a variable steering ratio controlling devicethat controls a steering angle of steered wheels by applying a drivingforce to a steering input; (2) an input torque obtaining unit thatobtains an input torque that is applied to a steering train of asteering system by external forces; (3) an operation torque calculatingunit that calculates an operation torque that a driver applies to asteering; and (4) a target steering angle adjusting unit that sets atarget steering angle of the variable steering ratio controlling devicesmaller when a first condition which is satisfied if the operationtorque is smaller than a first predetermined value and a secondcondition which is satisfied if the input torque is smaller than asecond predetermined value are both satisfied, than when at least one ofthe first and the second conditions is not satisfied.

A second aspect of the invention relates to a vehicular steering controlapparatus that includes a phase adjusting unit that, when a firstcondition which is satisfied if the operation torque is smaller than afirst predetermined value and a second condition which is satisfied ifthe input torque is smaller than a second predetermined value are bothsatisfied, sets a phase of a target steering angle of the variablesteering ratio controlling device to a phase that is different from aphase set when at least one of the first and the second conditions isnot satisfied. In this apparatus, the phase adjusting unit is providedinstead of the target steering angle adjusting unit of the apparatusaccording to the first aspect of the invention, which sets a targetsteering angle of the variable steering ratio controlling device smallerwhen a first condition which is satisfied if the operation torque issmaller than a first predetermined value and a second condition which issatisfied if the input torque is smaller than a second predeterminedvalue are both satisfied, than when at least one of the first and thesecond conditions is not satisfied.

A third aspect of the invention relates to a steering control method fora vehicle including a variable steering ratio controlling device thatcontrols a steering angle of steered wheels by applying a driving forceto a steering input. In the steering control method of the vehicle, aninput torque that is applied to a steering train of a steering system byexternal forces and an operation torque that the driver applies to asteering are obtained, and a target steering angle of the variablesteering ratio controlling device is set smaller when a first conditionwhich is satisfied if the operation torque is smaller than a firstpredetermined value and a second condition which is satisfied if theinput torque is smaller than a second predetermined value are bothsatisfied, than when at least one of the first and the second conditionsis not satisfied.

A fourth aspect of the invention relates to a steering control methodfor a vehicle including a variable steering ratio controlling devicethat controls a steering angle of steered wheels by applying a drivingforce to a steering input. In the steering control method of thevehicle, an input torque that is applied to a steering train of asteering system by external forces and an operation torque that a driverapplies to a steering are obtained, and when a first condition which issatisfied if the operation torque is smaller than a first predeterminedvalue and a second condition which is satisfied the input torque issmaller than a second predetermined value are both satisfied, a phase ofa target steering angle of the variable steering ratio controllingdevice is set to a phase that is different from a phase set when atleast one of the first and the second conditions is not satisfied.

The aforementioned vibration that occurs in the steering systemincluding the variable steering ratio controlling device when the driveris not holding the steering wheel tends to intermittently occur near theequilibrium point after the steering wheel returns to around the neutralposition. According to the invention, the vibration reduction control isperformed when the driver is not holding the steering wheel (i.e., theoperation torque is smaller than the first predetermined value) whilethe equilibrium point is reached (i.e., the input torque is smaller thanthe second predetermined value). It should be noted that the term“equilibrium point” herein indicates the point where the torque input tothe steering side and the reaction force from the steered wheels are inequilibrium.

The vibration reduction control according to the invention varies theresponse of the target steering angle, and in the first method,vibration is forcibly attenuated by setting the target steering anglesmaller. In the second method, attenuation of vibration is promoted bychanging the phase of the target steering angle from one which causesvibration. As the method to change the phase, for example, the filteringfrequency for calculating the target steering angle may be changed.

According to the invention, when the driver is not holding the steeringwheel around the equilibrium point, attenuation of vibration is promotedand occurrence of vibration is prevented by changing the response of thetarget steering angle. As a result, it is possible to effectivelyprevent or reduce vibration around the equilibrium point. In addition,the vibration reduction control is performed only when the driver is notholding the steering wheel while the equilibrium point is reached, andtherefore the accuracy of position control and the steering response arenot deteriorated. When there are some external disturbances, thevibration reduction control is not performed, and thus the positionaldeviation due to the external disturbances can also be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description ofpreferred embodiments with reference to the accompanying drawings,wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a view schematically showing the steering system that includesa vehicular steering control apparatus according to the first embodimentof the invention.

FIG. 2 is a control block diagram showing the configuration of the firstembodiment.

FIG. 3 is a graph showing a comparison between the result of thesteering control of the first embodiment and the result of steeringcontrol in which no vibration reduction control is performed.

FIG. 4 is a view schematically showing a steering system as modifiedfrom the first embodiment.

FIG. 5 is a control block diagram showing the configuration of thesecond embodiment.

FIG. 6 is a graph showing a comparison between the result of thesteering control of the second embodiment and the result of steeringcontrol in which no vibration reduction control is performed.

FIG. 7 is a view schematically showing the steering system that includesa vehicular steering control apparatus according to the third embodimentof the invention.

FIG. 8 is a control block diagram showing the configuration of the thirdembodiment.

FIG. 9 is a view schematically showing a steering system includingvariable steering ratio controlling device as modified from the firstembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will be hereinafter described indetail with reference to the attached drawings. In order to facilitateunderstanding of the description, the same reference numerals are usedfor the same elements in the drawings as much as possible, and samedescriptions of the elements are omitted.

As shown in FIGS. 1 and 2, a steering system functions to turn a rightfront wheel FR and a left front wheel FL, which are steered wheels, as asteering wheel 10 is turned. Rotation of a steering shaft that isconnected to the steering wheel 10 is converted to a linear motion inthe horizontal direction at a steering gear box 13, and is thentransmitted to the front wheels FR, FL through a tie rod 14.

The steering shaft includes an input shaft 11 connected to the steeringwheel 10 and an output shaft 12 connected to the steering gear box 13,which serves as a steering input. A gear mechanism 20 is providedbetween the input shaft 11 and the output shaft 12. An electric motor 21is connected to the gear mechanism 20. They constitute variable steeringratio controlling device.

An input angle sensor 30 and a torque sensor 31 are provided on theinput shaft 11. The input angle sensor 30 detects an operation angle θhof the steering wheel 10, and the torque sensor 31 detects an inputtorque T to the steering train of the steering system. The input torqueT is resultant force of external forces which are applying to thesteering train of the steering system other than an operation torque Th(as will hereinafter be described in detail). A motor output anglesensor 32 that detects a motor output shaft turning angle θp is providedon the motor output shaft of the motor 21. The turning angle of theoutput shaft 12 is the sum of θh and θp (θh+θp), and this value has apredetermined relationship with the steering angle of the front wheelsFR, FL.

A steering control unit 4 includes a CPU, a ROM, a RAM, an electriccircuit etc., and the output signals from the input angle sensor 30, thetorque sensor 31, the motor output angle sensor 32, and a vehicle speedsensor 50 that detects a vehicle speed V of the vehicle are input to thesteering control unit 4. The steering control unit 4 controls thedriving of the motor 21.

As shown in FIG. 2, the steering control unit 4 further includes atarget steering angle setting portion 41, an operation torque estimatingportion 42, a non-holding state detecting portion 43, an equilibriumdetecting portion 44, a target steering angle adjusting portion 45, aswitching portion 46, a compensator 23, a motor drive circuit 22,multipliers 47, 48, and an adder 49.

The target steering angle setting portion 41 sets a target output angleθpm based on the vehicle speed V detected by the vehicle speed sensor 50and the operation angle θh detected by the input angle sensor 30. Theturning angle θh+θp of the output shaft 12 and the steering angle δ ofthe steered wheels have a predetermined relationship defined by thecharacteristics of the steering gear box 13. Therefore, setting thetarget output angle θpm in accordance with the operation angle θh issynonymous with setting the target steering angle δm.

The operation torque estimating portion 42 estimates an operation torqueTh applied by a driver using an equation (1) that factors in the inputtorque T at the steering train of the steering system which is detectedby the torque sensor 31, and the operation angle θh which is detected bythe input angle sensor 30.

$\begin{matrix}{{Th} = {{{Ih}\frac{{\mathbb{d}^{2}\theta}\; h}{\mathbb{d}t^{2}}} + {{Ch}\frac{{\mathbb{d}\theta}\; h}{\mathbb{d}t}} + T}} & (1)\end{matrix}$

The reference symbol Ih denotes a moment of inertia at the steeringwheel 10, and Ch denotes a coefficient of viscous friction against therotation of the input shaft 11. The inertia moment Ih and the viscousfriction coefficient Ch have values specific to the type or structure ofeach steering system. Therefore, by obtaining the inertia moment Ih andthe viscous friction coefficient Ch in advance and storing them in theoperation torque estimating portion 42, it is possible to estimate theoperation torque Th from the input torque T, an angular velocity and anangular acceleration of the operation angle θh.

Then, based on the input torque T and the operation torque Th, thenon-holding state detecting portion 43 detects a non-holding state whichis the state where the driver is not holding the steering 10 and theequilibrium detecting portion 44 determines whether an equilibriumpoint, to be described later, is reached. Specifically, the non-holdingstate detecting portion 43 detects the non-holding state by comparingthe absolute value of the operation torque Th and a threshold value ε1.The threshold value ε1 is set to a value at and above which it isconsidered that the driver is holding the steering wheel 10controllably. That is, if the absolute value |Th| is equal to or largerthan the threshold value ε1, the non-holding state detecting portion 43determines that the non-holding state is not present and thereforeoutputs “0” to the multiplier 47. When the absolute value |Th| issmaller than the threshold value ε1, conversely, the non-holding statedetecting portion 43 determines that the non-holding state is presentand therefore outputs “1” to the multiplier 47. Meanwhile, theequilibrium detecting portion 44 determines whether an equilibrium pointis reached by comparing the absolute value of the input torque T and athreshold value ε2. The threshold value ε2 is set to a valuecorresponding to a state where the respective inputs to the steeringtrain of the steering system, i.e., the input from the driver to thesteering wheel 10 is equilibrates to the assist torque from the motor 21and the reaction force from the steered wheels FL, FR side. That is, ifthe absolute value |T| is equal to or larger than the threshold valueε2, the equilibrium detecting portion 44 determines that the equilibriumpoint is not reached, and outputs “0” to the multiplier 47. If theabsolute value |T| is smaller than the threshold value ε2, theequilibrium detecting portion 44 determines that the equilibrium pointis reached and outputs “1” to the multiplier 47.

The output values (0 or 1) from the non-holding state detecting portion43 and the equilibrium detecting portion 44 are multiplied by themultiplier 47. In this way, “1” is output to the switching portion 46only when the non-holding state is present and the equilibrium point isreached. When either or both of the these conditions is not satisfied,such as when the driver is holding the steering wheel 10, or when thedriver is not holding the steering wheel 10 but the equilibrium point isnot reached, “0” is output to the switching portion 46.

The switching portion 46 is connected to the output side of the targetsteering angle setting portion 41, and the operation of the switchingportion 46 is controlled by the output from the multiplier 47. Morespecifically, when the output value of the multiplier 47 is 0, theswitch is set to the upper position in the switching portion 46, as seenin the drawing, and θpm output from the target steering angle settingportion 41 is directly input to a positive input terminal of the adder49. When the output value of the multiplier 47 is 1, the switch is setto the lower position in the switching portion 46, as seen in thedrawing. A multiplier 48 is provided between the positive input terminalof the adder 49 and the switching portion 46, and θpm′ calculated by themultiplier 48 is input to the positive input terminal of the adder 49.The output value of a target steering angle adjusting portion 45 isinput to the multiplier 48. The target steering angle adjusting portion45 sets a gain G in accordance with the input torque T detected by thetorque sensor 31. The gain G is 1 when the absolute value of the inputtorque T is equal to the threshold value ε2, and approaches 0 as theinput torque T decreases. That is, θpm′which is the output value of themultiplier 48, is obtained by multiplying G with θpm (G×θpm), andtherefore it approaches 0 as the input torque T decreases.

θp output from the motor output angle sensor 32 is input to a negativeinput terminal of the adder 49. Therefore, when it is determined thatthe driver is not holding the steering wheel 10 and the equilibriumpoint is reached, the output from the adder 49 is θpm′-θp. In othercases, the output value of the adder 49 is θpm-θp. That is, thedifference between the adjusted target output angle and the actualoutput angle is output to the compensator 23.

The compensator 23 calculates the driving amount of the motor 21 inaccordance with the difference between the target output angle and theactual output angle, and transmits the calculated driving amount to themotor drive circuit 22. The motor drive circuit 22 then drives the motor21 according to the driving amount. In this way, by bringing the actualoutput angle closer to the target output angle, the steering angle iscontrolled to be come a desired angle.

FIG. 3 is a graph showing a comparison between the result of thesteering control of the first embodiment and the result of steeringcontrol in which no vibration reduction control is performed, when thedriver takes his or her hands off the steering wheel 10 so as to allowthe steering wheel 10 to turn back to the neutral position after turningthe steering wheel 10 to the maximum steering angle. In the graph, θpm1shows changes with time in the target output angle and θp1 shows changeswith time in the actual output angle during the steering control with novibration reduction control. On the other hand, θpm2 shows changes withtime in the target output angle and θp2 shows changes with time in theactual output angle during the steering control of the embodiment.

Unavoidably, the actual output angle changes with delay in response to achange in the target output angle due to, for example, deformation andfriction in the steering train of the steering system. Because of suchdelay, vibration occurs around the neutral position during the steeringcontrol with no vibration reduction control. To counter this, in thesteering control of the embodiment, when the steering reaches around theneutral position, which corresponds to the equilibrium point, the targetoutput angle is reduced to reduce the target steering angle (see θpm2).As a result, the operation amount of the motor 21 is reduced, so thatvibration is effectively suppressed (see θp2). In this case, the controlgain is substantially the same as used in a regular control, andtherefore good steering response is maintained. Meanwhile, in the casewhere an external force is acting on the steering train of the steeringsystem and thus the equilibrium point is not reached, the regularcontrol is performed, and therefore the accuracy of position control andthe response can be maintained. Accordingly, the steering controlapparatus of the embodiment improves the overall steering feeling.

In the first embodiment, the torque sensor 31 is provided on the inputshaft 11, but it may alternatively be provided on the output shaft 12 asshown in FIG. 4. In this case, too, it is possible to estimate theoperation torque Th based on the equation (1).

FIG. 5 is a block diagram showing the configuration of a vehicularsteering control apparatus according to the second embodiment of theinvention. This apparatus is used as a steering control apparatus forthe steering system shown in FIG. 1 or FIG. 4. In the steering controlunit 4 of the first embodiment shown in FIG. 2, as mentioned above, theswitching portion 46 is provided between the target steering anglesetting portion 41 and the adder 49, and it switches, in accordance withthe output value of the multiplier 47, between the mode where the outputof the target steering angle setting portion 41 is directly input to theadder 49 and the mode where the output of the target steering anglesetting portion 41 is adjusted using the output value of the targetsteering angle adjusting portion 45. Meanwhile, in a steering controlunit 4 a of the second embodiment, the switching portion 46 is providedbetween the input angle sensor 30 and the target steering angle settingportion 41, and a filter for filtering the input angle signals input tothe target steering angle setting portion 41 is switched between a firstfilter 51 and a second filter 52.

The first filter 51 and the second filter 52 have different filterfrequencies, therefore the phase characteristic of the target outputangle set by the target steering angle setting portion 41 is differentdepending on which of the first filter 51 or the second filter 52 isused. The use of such different phase characteristics, in particular,makes it possible to suppress resonance caused by a phase differencebetween the target output angle input to the adder 49 and the measuredoutput angle when vibration occurs. The phase characteristic of thetarget output angle obtained with the second filter 52 can be madedifferent from that obtained with the first filter 51 by setting thefilter frequency of the second filter 52 higher or lower than the filterfrequency of the first filter 51.

FIG. 6 is a graph showing a comparison between the result of thesteering control of the embodiment and the result of steering control inwhich no vibration reduction control is performed, when the driver takeshis or her hands off the steering wheel 10 so as to allow the steeringwheel 10 to turn back to the neutral position after turning the steeringwheel 10 to the maximum steering angle. In the graph, θpm3 shows changeswith time in the target output angle and θp3 shows changes with time inthe actual output angle during the steering control with no vibrationreduction control. On the other hand, θpm4 shows changes with time inthe target output angle and θp4 shows changes with time in the actualoutput angle during the steering control of the embodiment.

In the steering control with no vibration reduction control, whenvibration occurs, the vibration cannot be reduced because the phaserelationship between the target output angle and the actual output anglepromotes resonance, and as a result, the vibration continues for a longtime. In order to prevent such resonance, during the steering control ofthe embodiment, the phase characteristic of the target output angle isselectivelyset. Therefore, when vibration is occurring, the vibration iseffectively and immediately attenuated. Similarly to the firstembodiment, in the case where an external force is acting on thesteering train of the steering system and thus the equilibrium point isnot reached, i.e., one of the conditions for performing the vibrationreduction control is not satisfied, the regular control is performed,and therefore the accuracy of position control and the response are notdeteriorated. Accordingly, the steering control apparatus of theembodiment improves the overall steering feeling.

FIG. 7 is a view schematically showing the configuration of a steeringsystem incorporating a steering control apparatus according to the thirdembodiment of the invention, and FIG. 8 is a block diagram showing theconfiguration of the steering control apparatus. The steering system inthe third embodiment, as shown in FIG. 7, differs from those shown inFIGS. 1 and 4 in that the torque sensor 31 is removed and a motorcurrent sensor 24 that detects current supplied to the motor 21 isprovided.

As shown in FIG. 8, a steering control unit 4 b has basically the sameconfiguration as the steering control unit 4 in the first embodiment. Inthe steering system shown in FIG. 2, as mentioned above, the torquesensor 31 serves as an input torque obtaining portion that detects theinput torque T that is applied to the steering train of the steeringsystem by external forces, and the output from the torque sensor 31 isdirectly input to the operation torque estimating portion 42 and theequilibrium detecting portion 44. In contrast, in the steering system ofthe third embodiment shown in FIG. 8, a torsional force estimatingportion 25 serves as the input torque obtaining portion that receives acurrent value Ir output from the motor current sensor 24 and estimatesthe input torque T based on it is provided.

In the steering system in FIG. 7, the steering wheel 10 does not receivethe torque generated by the motor 21. Therefore, the torsional forceacting on the input shaft 11 is estimated using an equation (2) whichfactors in the inertia moment Im of the motor 21 and the gear mechanism20 and the viscous friction coefficient Cm.

$\begin{matrix}{T = {{KmGmIr} - {{Im}\frac{{\mathbb{d}^{2}\theta}\; p}{\mathbb{d}t^{2}}} - {{Cm}\frac{{\mathbb{d}\theta}\; p}{\mathbb{d}t}}}} & (2)\end{matrix}$

Km denotes the torque constant of the motor 21, and Gm denotes thetransmission ratio of the variable steering ratio control mechanism. Theoperation torque applied by the driver can be estimated by assigning theresult of the equation (2) into the equation (1).

In the third embodiment, similar effects and advantages to those in thefirst embodiment can be obtained. The steering control apparatusaccording to the third embodiment, for example, may be advantageouslyemployed in a hydraulic power steering apparatus. On the other hand, thesteering control apparatus according to the first embodiment may beadvantageously employed in an electric power steering apparatus.

The variable steering ratio control mechanism is not limited to thosedescribed above, but may be constructed such that, for example, thesteering input directly receives torque from the electric motor as shownin FIG. 9. In this case, since the equation of motion for the steeringsystem is different from those in the foregoing embodiments, differentcalculation methods are used for estimating the operation torque of thedriver and the input torque. However, the basic concept is the same.

While the invention has been described with reference to what areconsidered to be preferred embodiments thereof, it is to be understoodthat the invention is not limited to the disclosed embodiments orconstructions. On the contrary, the invention is intended to covervarious modifications and equivalent arrangements. In addition, whilethe various elements of the disclosed invention are shown in variouscombinations and configurations, which are exemplary, other combinationsand configurations, including more, less or only a single element, arealso within the spirit and scope of the invention.

1. A vehicular variable steering ratio control device that controls asteering angle of steered wheels by applying a driving force to asteering input, comprising: an input torque obtaining portion thatobtains an input torque that is applied to a steering train of asteering system by external forces; an operation torque estimatingportion that calculates an operation torque that a driver applies to asteering; a target steering angle adjusting portion that sets a targetsteering angle of the variable steering ratio control device smallerwhen a first condition which is satisfied if the operation torque issmaller than a first predetermined value and a second condition which issatisfied if the input torque is smaller than a second predeterminedvalue are both satisfied, than when at least one of the first and thesecond conditions is not satisfied; and a motor that applies drivingforce to the steering input to the steered wheels based on the settarget steering angle.
 2. The vehicular variable steering ratio controldevice according to claim 1, wherein the input torque obtaining portionincludes a torque detector that detects the input torque.
 3. Thevehicular variable steering ratio control device according to claim 1,further comprising an electric motor that applies the driving force tothe steering input and a motor current detector that detects a value ofcurrent supplied to the electric motor, wherein the input torqueobtaining unit includes a torsional force estimating portion thatestimates the input torque based on the detected current value.
 4. Thevehicular variable steering ratio control device according to claim 1,wherein the operation torque estimating portion estimates the operationtorque based on the input torque, a steering angular velocity, and asteering angular acceleration.
 5. The vehicular variable steering ratiocontrol device according to claim 1, wherein the target steering angleadjusting portion sets the target steering angle smaller as the inputtorque becomes smaller.
 6. A steering control method for a vehicleincluding a variable steering ratio control device that controls asteering angle of steered wheels by applying a driving force to asteering input, comprising: obtaining an input torque that is applied toa steering train of a steering system by external forces; calculating anoperation torque that a driver applies to a steering; setting a targetsteering angle of the variable steering ratio control device smallerwhen a first condition which is satisfied if the operation torque issmaller than a first predetermined value and a second condition which issatisfied if the input torque is smaller than a second predeterminedvalue are both satisfied, than when at least one of the first and thesecond conditions is not satisfied; and applying a driving force to thesteering input to the steered wheels based on the target steering angle.7. The steering control method for the vehicle according to claim 6,wherein the operation torque is estimated based on the input torque, asteering angular velocity, and a steering angular acceleration.